Rheometer

ABSTRACT

Apparatus and a method for investigating dynamic rheological properties of a viscoelastic medium involves oscillating at least one shear wave generation surface in a container containing the viscoelastic medium such that shear waves generated thereby impinge on two spaced shear wave receiving surfaces, the receiving surfaces being spaced by different distances from the source of shear waves impinging thereon. Transducers are provided for the shear wave receiving surfaces capable of generating output signals dependent on the variations in physical parameters of heating the wave receiving surfaces induced by the oscillating shear wave generation surfaces.

BACKGROUND OF THE INVENTION

The present invention relates to a device for investigating theproperties of a viscoelastic medium, and more particularly, to arheometer for measuring the dynamic rheological properties of gels,melts, polymer solutions, and other highly viscous media.

Conventional dynamic rheological techniques involve the forcedgeneration of shear waves of varying magnitude in the medium underinvestigation and the recording of displacements of a wave-receiving andreflecting surface using a suitable transducing device. Phase andmagnitude relationships between corresponding force and displacementwaveforms are then analysed to determine the rheological properties ofthe medium. The major disadvantage of such techniques lies in the factthat accurate determination of the dynamic stresses and strains atmeasuring geometry boundaries is required to produce meaningful results.Such accurate determination is difficult to achieve at high frequencies.

SUMMARY OF THE INVENTION

The present invention enables a technique to be used relying on theutilisation of phase and magnitude relationships between correspondingdisplacement waveforms (relative to a common forcing source) generatedby transducers connected to two wave receivers/reflectors disposed atdifferent distances from the source, to determine the shear wavevelocity and attenuation.

According to a first aspect of the invention, there is providedapparatus for investigating dynamic rheological properties of aviscoelastic medium comprising:

(a) a container for said viscoelastic medium;

(b) a driven member oscillatable about an axis;

(c) at least one shear wave generating surface for generating shearwaves in said medium on oscillation of said driven member;

(d) a first shear wave receiving surface facing one said wave generatingsurface;

(e) a second shear wave receiving surface facing a second said wavegenerating surface, which may be the same as, or different to, said onewave generating surface, said second shear wave receiving surface beingspaced from said second wave generating surface by an amount differentto the spacing between said first shear wave receiving surface and saidone shear wave generating surface; and

(f) at least one transducer connected to each wave receiving surface,for sensing variations in physical parameters affecting said wavereceiving surfaces induced by oscillation of said driven member, andcapable of generating an output signal.

Preferably two shear wave generating surfaces are provided, in whichcase it is preferable that each of the shear wave receiving surfacessubstantially faces a respective opposed shear wave generating surface.It is further preferred that each of the shear wave generating surfacesshould be substantially planar and should oscillate in a respectiveplane, the respective planes being substantially parallel to acorresponding substantially planar receiving surfaces.

Preferably, means for adjusting the separation between at least one ofthe wave generating surfaces and a respective wave receiving surface isprovided. The adjusting means may be a micrometer connected to at leastone of the wave generating surfaces.

It is preferred that the oscillatable member should be driven by asuitably arranged set of Helmholtz coils energised by a voltage suppliedby a signal generator. It is further preferred that the voltage suppliedshould be sinusoidal.

The oscillations are preferably of relatively small amplitude, such thatlinear viscoelastic behaviour is invoked in the medium and the frequencyof oscillation preferably lies in the range of 50 Hz to 3 kHz.

The transducers may be of piezo-crystal type or preferably of the noncontacting inductive or capacitive type.

Further adjustment means may be provided enabling the driving member tobe movable in directions substantially normal to, and substantiallyparallel to, the planes of the wave receiving surfaces.

It is preferred that the output signal from each transducer should bepassed to signal processing means (as shown by reference numeral 17 inFIG. 1) arranged to provided a signal representative of the rheologicalproperties of the test viscoelastic medium, determined according tocomputed values derived from the measured response from the respectivetransducers. The resulting signal may be processed by signalconditioning means for output display purposes.

According to a second aspect of the invention, there is provided amethod of investigating dynamic rheological properties of a viscoelasticmedium by utilising output signals from at least two transducers, eachof which is connected to a respective shear wave receiving surface,which surfaces are disposed substantially within the medium underinvestigation such that the distance between one said receiving surfaceand a shear wave generation surface is different to the distance betweena second said receiving surface and a corresponding shear wavegeneration surface.

A particular embodiment of the invention will be further described, byway of example only, with reference to the accompanying drawings inwhich:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of apparatus according to the invention;

FIG. 2 is a graphical representation of the transducer output.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIG. 1, a driving rod 11, which has secured to itslower end opposed shear wave generating plates 15, is attached to ajewelled bearing between a set of Helmholtz coils 12 and suspendedbetween two steel discs 2; the latter plates are in direct contact withpiezo-crystals 3 which serve to monitor deflection of the discs 2 asshear waves generated at the plates 15 impinge upon them. The coils 12and attached rod 11 are mounted at the base of a plastics pillar 10 by agimballed mount, allowing a wide degree of manipulation of bothhorizontal and vertical displacements of an electromagnetic driveassembly 9. The relative dispositions of generating plates 15 and discs2 are achieved by micrometer positioning 5 and 6 and by the fact thatthe base of pillar 10 is movable relative to the test cell 1.

A voltage supplied by a signal generator (not shown) to the coils causesthe rod 11 to perform small amplitude sinusoidal oscillations atcontrolled frequencies and displacements. Shear waves thereby generatedat the plates 15 pass through the viscoelastic test medium 1 present ina chamber 16, and are detected at the surfaces of the discs 2. As thediscs 2 are at unequal distances from 11 (the common source of thewaves) a time-lag (or phase difference) is detected between the outputsfrom the piezo-crystals 3. knowledge of the unequal path lengths andtime difference in arrival provides the wave velocity, as required.Calibration of transducer outputs leads to a knowledge of the relevantshear strain amplitude involved in the measurement, which may be variedby adjusting the driving voltage amplitude. The usable frequency rangeis 80 Hz to a few kilohertz.

Referring to FIG. 2, output waveform A is produced by the plate 15 whichis nearer to rod 11, at a distance x from the rod. Output waveform B isproduced by the plate which is at a larger distance (x+Δx) away. Thevelocity of the shear waves v is then given by:

    v=Δx/Δt

Attenuation of the shear waves is measured directly by standardtechniques of noting the variation of transducer response as path lengthor frequency are varied. Thus, knowing v and the critical dampinglength, the dynamic moduli (the rigidity modulus and the loss modulus)can be obtained by known calculations.

We claim:
 1. Apparatus for investigating dynamic rheological propertiesof a viscoelastic medium, comprising:(a) a container for theviscoelastic medium; (b) a driving member ocsillatable about an axis;(c) at least one shear wave generating surface supported on said drivingmember for generating shear waves in the viscoelastic medium uponoscillation of said driving member; (d) a first shear wave receivingsurface arranged to be capable of facing and being spaced a firstdistance from one of said at least one shear wave generating surface;(e) a second shear wave receiving surface arranged to be capable offacing and being spaced a second distance from one of said at least oneshear wave generating surface, said second distance being different fromsaid first distance; and (f) a transducer connected to each wavereceiving surface, for sensing variations in physical parametersaffecting said wave receiving surfaces induced by oscillation of saiddriven member, and capable of generating an output signal.
 2. Apparatusaccording to claim 1, which comprises two discrete said shear wavegenerating surfaces.
 3. Apparatus according to claim 2, wherein each ofthe shear wave generating surfaces is substantially planar and oscillatein a respective plane, the respective plane being substantially parallelto corresponding substantially planar receiving surfaces.
 4. Apparatusaccording to claim 3, wherein each of the shear wave receiving surfacessubstantially faces a respective opposed shear wave generating surface.5. Apparatus according to claim 3, further comprising adjusting meanscoupled to one of said first shear wave receiving surface, said secondshear wave receiving surface and said at least one shear wave generatingsurface for adjusting the separation between said at least one shearwave generating surface and one of said shear first wave receivingsurface and said second shear wave receiving surface.
 6. Apparatusaccording to claim 3, further comprising adjustment means coupled tosaid driving member for moving said driving member in directionssubstantially normal to, and substantially parallel to, planes of saidwave receiving surfaces.
 7. Apparatus according to claim 2, wherein eachof the shear wave receiving surfaces substantially faces a respectiveopposed shear wave generating surface.
 8. Apparatus according to claim2, further comprising adjusting means coupled to one of said first shearwave receiving surface, said second shear wave receiving surface andsaid at least one shear wave generating surface for adjusting theseparation between said at least one shear wave generating surface andone of said shear first wave receiving surface and said second shearwave receiving surface.
 9. Apparatus according to claim 2, furthercomprising adjustment means coupled to said driving member for movingsaid driving member in directions substantially normal to, andsubstantially parallel to, planes of said wave receiving surfaces. 10.Apparatus according to claim 1, wherein each of the shear wave receivingsurfaces substantially faces a respective opposed shear wave generatingsurface.
 11. Apparatus according to claim 10, further comprisingadjusting means coupled to one of said first shear wave receivingsurface, said second shear wave receiving surface and said at least oneshear wave generating surface for adjusting the separation between saidat least one shear wave generating surface and one of said shear firstwave receiving surface and said second shear wave receiving surface. 12.Apparatus according to claim 10, further comprising adjustment meanscoupled to said driving member for moving said driving member indirections substantially normal to, and substantially parallel to,planes of said wave receiving surfaces.
 13. Apparatus according to claim1, further comprising adjusting means coupled to one of said first shearwave receiving surface, said second shear wave receiving surface andsaid at least one shear wave generating surface for adjusting theseparation between said at least one shear wave generating surface andone of said shear first wave receiving surface and said second shearwave receiving surface.
 14. Apparatus according to claim 13, furthercomprising adjustment means coupled to said driving member for movingsaid driving member in directions substantially normal to, andsubstantially parallel to, planes of said wave receiving surfaces. 15.Apparatus according to claim 1, further comprising adjustment meanscoupled to said driving member for moving said driving member indirections substantially normal to, and substantially parallel to,planes of said wave receiving surfaces.
 16. A method of investigatingdynamic rheological properties of a viscoelastic medium by utilizingoutput signals from at least two transducers, said methodcomprising:disposing a shear wave generation surface substantiallywithin the viscoelastic medium; operatively connecting each of thetransducers to a respective shear wave receiving surface, each of thetransducers providing output signals; and disposing each of said shearwave receiving surfaces substantially within the viscoelastic mediumsuch that the distance between one of said receiving surfaces and saidshear wave generation surface is different from the distance between asecond of said receiving surfaces and said shear wave generationsurface, wherein the output signals of the transducers are utilized toobtain a rheological property of the viscoelastic medium.
 17. A methodaccording to claim 16, wherein said shear wave generation surfaceoscillates in said viscoelastic medium.
 18. A method according to claim17, wherein said shear wave generation surface oscillates generallysinusoidally at a frequency in the range 50 hz to 3 Khz.
 19. A methodaccording to claim 17, wherein the output signal from each transducer ispassed to signal processing means arranged to provided a signalrepresentative of the rheological properties of the test viscoelasticmedium, determined according to computed values derived from themeasured response from the respective transducers.
 20. A methodaccording to claim 18, wherein the output signal from each transducer ispassed to signal processing means arranged to provided a signalrepresentative of the rheological properties of the test viscoelasticmedium, determined according to computed values derived from themeasured response from the respective transducers.
 21. A methodaccording to claim 16, wherein the output signal from each transducer ispassed to signal processing means arranged to provided a signalrepresentative of the rheological properties of the test viscoelasticmedium, determined according to computed values derived from themeasured response from the respective transducers.
 22. A methodaccording to claim 16, further comprising: disposing a second shear wavegeneration surface substantially within the viscoelastic medium.